1. Technical Field
The present invention relates to a low noise amplifier structure dapted to amplify an input signal applied to input terminals thereof to thereby generate an output signal at an output thereof, said structure including:
a differential difference amplifier having said output and a plurality of input terminals arranged in a first and in a second differential input pair, said first differential input pair comprising a first input terminal of a first polarity type and a second input terminal of a second polarity type opposite to said first polarity type, and said second differential input pair comprising a third input terminal of said first polarity type and a fourth input terminal of said second polarity type; and PA1 a feedback structure including first feedback means (R1a, C1a; R1b, C1b) coupled between said output and at least one input terminal among said plurality and second feedback means (R2a, R2a'; R2b, R2b') coupled between said one input terminal and a common node, said input signal being applied to a set of two input terminals among said plurality and different from said one input terminal. PA1 that said second feedback means comprises a third feedback circuit coupling said first input terminal to said common node, and a fourth feedback circuit coupling said fourth input terminal to said common node. PA1 a first pair of switches adapted to connect said third and fourth output terminals of said second differential difference amplifier to said first and fourth input terminals of said first differential difference amplifier; PA1 a second pair of switches adapted to connect said third and fourth output terminals of said second differential difference amplifier to said common node of said third and fourth feedback circuits; and PA1 at least a third pair of switches, each switch of said third pair being connected across a resistor of a distinct one of said resistive means.
2. Discussion of Related Art
Such a low noise amplifier structure is already known in the art, e.g. from the article "A 3.3-V 800-nVrms Noise, Gain-Programmable CMOS Microphone Preamplifier Design Using Yield Modeling Technique" by G. Nicollini et al. of SGS-Thomson Microelectronics, and published in the IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 28, NO. 8, Aug. 1993, pages 915 to 921. Therein, different amplifier arrangements are compared in order to obtain a low noise amplifier structure with a high resistive input impedance and a wide input common mode range. It has been proved that the best structure is based on the use of a "Differential Difference Amplifier" (DDA). In this best known structure, the input signal is applied to the first differential input pair of the amplifier. The first feedback means of the feedback structure which controls the gain of the amplifier, is constituted by a first resistor connected between the output and an input terminal of the second differential input pair, while the second feedback means is constituted by a second resistor connected between the latter input terminal and the common node which, in this case, is the ground. The other input terminal of the second differential input pair is also connected to the common node, i.e. to the ground.
A problem with this known low noise single ended amplifier structure is that an unacceptable distortion appears in the output signal when the amplitude of the input signal becomes relatively high, e.g. in the range of 1 Vpp. In the article, the amplitude of the input signal is limited to 100 mV.
The reason thereof is that the dynamic range of a single differential input pair is insufficiently high. This dynamic range can for instance be enlarged by decreasing the Width-to-Length ratio (W/L) of the input transistors of the differential input pair. However, smaller transistors result in more input referred noise which is not recommended for low noise amplifiers.